My invention relates to the electrical protection of communication circuits and, more particularly, to improved fusing for current overload protection.
Occasional faults in telephone lines outside of customer premises occur due to lightning strikes and other phenomenon. In order to safeguard a customer telephone station from such hazards, a station protector located on the customer premises is used to interface the outside telephone line to the station line. A station protector usually employs known carbon-block, air-gap, or gas-tube devices for voltage overload protection. Current overload protection is typically effected by fusing. A station protector fuse must be able to reliably interrupt an alternating current of at least 350 amperes at 3000 volts. When a fuse wire melts, a gap results which is susceptible to a potentially destructive power arc. A power arc occurs when the path across the gap ionizes. The voltage at which ionization occurs decreases as the length of the gap decreases. The maximum voltage rating of a fuse thus corresponds nominally to the length of the gap.
Due to manufacturing and installation considerations, it is desirable to shrink the fuse length without lowering the voltage rating of the fuse. One arrangement encloses the fuse wire inside a tube filled with sand. The sand acts as an arc suppressant so that a fuse length of about four inches offers the aforementioned degree of electrical protection. Modern construction trends toward out-of-sight equipment installation, however, now demand even more compactly designed fused station protectors. Contemporary fused station protectors also have increased manufacturing costs because of the many mounting parts required for replaceable fuses. Replaceable fuses are unnecessary, however, since field experience has shown that the occurrence of a blown fuse is rare; it is economically desirable to treat the entire protector as an expendable unit. It is therefore an object of the invention to provide a compact, expendable fused station protector.